1. Field of the Invention
The present invention relates to a laser welding method for achieving high joining strength between members of the semiconductor device.
2. Description of the Related Art
FIG. 15 shows a construction of an ordinary power semiconductor module 500. This power semiconductor module 500 comprises: heat radiating base 51, a direct copper bonding (DCB) substrate 54 having a rear surface conductor pattern 53 fixed to the heat radiating base 51 through a joining material 52 such as solder, a semiconductor chip 57 fixed onto a front surface conductor pattern 55 of the DCB substrate 54 through a joining material 56 such as solder, a terminal-inserted type resin casing 58 fixed to the heat radiating base 51, and terminals 59, 60, and 61 fixed to the terminal-inserted type casing 58.
The power semiconductor module 500 further comprises a surface electrode and a gate pad (which are not depicted) of the semiconductor chip 57, and bonding wires 62 for connection among the surface electrodes, the gate pad, the front surface conductor pattern 55, and terminals 59, 60, and 61. The power semiconductor module 500 contains sealing gel material 63 filling the terminal-inserted type resin casing 58.
The DCB substrate 54 is composed of a ceramic insulated substrate 54a, a rear surface conductor pattern 53, and a front surface conductor pattern 55.
In a power semiconductor module 500 today, high current carrying capacity and small size are required. Thus, the semiconductor chip 57 of the module is used at a high current density for carrying heavy current and for down-sizing. Consequently, one of the most important issues with the recent power semiconductor module 500 is effective radiation of heat generated in the semiconductor chip 57 in order to secure reliability in high power operation.
To solve this issue, a lead-frame, which has a larger cross-sectional area of the current path and greater heat capacity than those of conventional bonding wires 62, is used in a known power semiconductor nodule 57 in place of the bonding wires 62. The lead-frame is partly utilized for wiring terminals and externally leading-out terminals. The wiring terminal is joined to the surface of the upper main electrode of the semiconductor chip 57 and utilized for a heat removing path. Thus, the heat generated in the semiconductor chip 57 is removed also from the upper surface thereof, as well as from the lower surface.
Patent Document 1 discloses a means for improving heat removal and averaging the temperature distribution on the chip. The means is composed of a heat spreader with high thermal conductivity joined on the upper surface of the semiconductor chip to spread the heat in the central region of the semiconductor chip toward surroundings.
In the wiring construction as described above, the joining process between the externally leading out terminal and the wiring terminal using a lead frame is carried out by means of laser welding as disclosed in Patent Document 2 as well as resistance welding and ultrasonic bonding. The laser welding joins two materials to be welded by first melting the materials with the energy of laser light and then cooling down them to a solid state. The laser welding has a characteristic feature, unlike the resistance welding and ultrasonic bonding, that the welding process can be carried out without contacting the joining apparatus the materials to be welded. Known laser welding techniques include seam welding and spot welding. In the seam welding, the material to be welded melts by continuously irradiating laser light, and in the spot welding, the material to be welded melts by irradiating high power pulse laser light on a spot of the material. Because the spot welding uses laser light with a high energy density, deep weld penetration is achieved as compared with the seam welding. Thus, two sheets of relatively thick metallic plates can be welded putting one on the other.
Patent Document 3 discloses laser welding at a plurality of places on lead-frame terminal and positioning of the material to be welded by preliminarily forming protruding parts and recessed parts at the position of laser welding of two sheets of materials.
Patent Document 4 discloses laser welding in which the laser welding place of the lead frame terminal is exposed out of the sealing resin.
Patent Document 5 discloses laser welding in which the lead frame to be welded is pressed and the vicinity of laser welding spot is pressed by a heat removing probe with a configuration of a circular or rectangular column.
Patent Document 6 discloses plating on laser welding places and covering the semiconductor chip with sealing resin to prevent sputtered particles from scattering onto the semiconductor chip.
Patent Document 7 discloses use of plating material of nickel, palladium, and gold.
Patent Document 8 discloses locally reducing the thickness of a lead frame to decrease rigidity of the lead frame.
Patent Document 9 discloses laser welding conducted through a light transmitting window of a slit.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-307058
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-096135
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2001-045634 (FIGS. 1 through 7, in particular)
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2007-265962 (FIG. 2 and FIG. 8, in particular)
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 2009-190067 (Paragraph 0007, paragraph 0008, and paragraph 0017, in particular)
[Patent Document 6] International Patent Application Publication WO 2009-081723 (FIG. 1, FIG. 8, and Paragraph 0024, in particular)
[Patent Document 7] Japanese Unexamined Patent Application Publication No. 2011-077278 (Paragraph 0026, in particular)
[Patent Document 8] Japanese Unexamined Patent Application Publication No. 2008-098585 (Paragraph 0043, in particular)
[Patent Document 9] Japanese Unexamined Patent Application Publication No. H07-014969
Power semiconductor modules 500, which can control a heavy current, are used in a wide range of industrial fields including motor control of electric vehicles and controllers for motor driving. In application in those fields, further down-sizing and enhancement of reliability of the power semiconductor module 500 are required. Down-sizing of the semiconductor chip 57 is being promoted in order to cope with down-sizing of the semiconductor module. This needs high current density in the semiconductor chip 57. Thus, effective removal of the heat generated in the semiconductor chip 57 is a key issue for ensuring high reliability in high power operation.
When a wiring terminal is joined with the main electrode surface of a semiconductor chip 57 and the wiring terminal is laser welded with the externally leading out terminal for the purpose of improving heat dissipation, the joined condition of the laser welding part affects the long term reliability of heat dissipation property and current carrying characteristic at the joint part.
If the joining area of the laser welded part is small, not only the heat removal path is narrow, but the welded part would become a spot of abnormal heating on running a heavy electric current.
A semiconductor device like the power semiconductor module 500 may suffer from thermal deformation in the whole module or a part thereof due to operation in an environment of violent temperature change and caused by Joule heating due to heavy current through the device. This thermal deformation can cause stress concentration at the laser welded part.
The main factors that impair stability of the joint condition of the laser welded part are the positional shift between the externally leading out terminal and the wiring terminal, and the gap between these terminals. The positional shift and the gap occur depending on a variety of factors including dimensional tolerance of these members (the externally leading out terminal and the wiring terminal), tolerance in assembling process, ease of assembling work, and thermal deformation of the module. The positional shift between the overlapped terminals may cause penetration of laser light through the terminal or damage of the module. If the gap between the terminals is too large, the thermal energy by the laser light is not transferred to the lower terminal, causing failure of joining.
Therefore, it is important for obtaining a stable laser welding part to minimize the positional shift and a gap between the terminals in the construction for laser welding the externally leading out terminal and the wiring terminal.
The Patent Documents 1 through 9 all fail to mention about using a laser welding jig having a plurality of pressing parts and laser welding while pressing one of the overlapped members to be welded.